A filter assembly typically includes at least one filter element which removes one or more undesirable substances from a fluid such as a liquid. For example, the filter element may be used to remove particulate or colloidal matter from the fluid. It may also be used to remove certain chemicals, minerals, or ions from the fluid. Consequently, filtration or filtering, as used in this application, includes the removal of particulate or colloidal matter from a fluid, for example, by sieving or trapping the matter, and/or the purification of a fluid, for example, by ion exchange, demineralization, or sorption.
During use, the filter element may become clogged or fouled by the particulate or colloidal matter in the fluid being filtered. This increases the pressure drop across the filter element and reduces its utility. Therefore, it is desirable to be able to clean the filter element to remove the accumulated matter.
There are also filter elements which it may be desirable to clean even when they have not become fouled. An example of such a filter element is a so-called precoat filter element, which has a layer of particles, such as ion exchange resin particles, coated on the upstream surface of the filter element prior to use. This layer, referred to as a precoat layer, is able to filter various substances from the fluid being filtered. However, over time, the precoat layer becomes fouled or depleted of its capacity and its ability to remove substances is reduced. At this point, it is desirable to clean the filter element so as to remove the old precoat layer and then replace it with a new precoat layer.
One method of cleaning a filter element is known as backwashing. In this method, a cleaning liquid (such as water), referred to as a backwash liquid, is forced at a high rate of flow, pressure, and/or volume through the filter element in the opposite direction from the direction of fluid flow during normal filtering operations. Typically, the flow of backwash liquid through the filter element is produced by gas under pressure acting on the backwash liquid. However, conventional backwashing arrangements for filter elements tend to be complicated and may require a large quantity of backwash liquid.